Hi, 99 996 3.4 H6 C2, 34,000 miles, changed Injectors, upgraded coil packs, new plugs, new MAF & O2 sensors.
Was running well prior to the work however I sensed a slight miss at idle. No CEL or anything on the scan.
It’s now idling/running better than ever however when starting from cold RPM goes straight to 2000 and immediately settles down to 840, when hot it cranks a number of times before starting, sometimes needs throttle input. No fuel leaks.
Thank you for the help in advance.

 

Check all the vacuum lines.

 

Check the oil filler pipe for cracks.

 

This bit is normal.

 

As Mike Murphy said.
Difficult hot start: Check whether the fuel vent valve is shut and does not allow fuel vapours through. Easiest check is by clamping the hose shut between valve and throttle body when engine hot and then start. If no add. throttle required then it's the valve (or the wiring to it). If not look further.

 

@hardtailer I ended up putting a new Fuel Vent Valve (Regeneration Valve) in yesterday and the starting problem is still there.
it starts fine when cold and starts fine when hot within 30 minutes of shut down, however after 45 minutes won’t start first or second try but will eventually start.

 

Crank position sensor is notorious for hot no start intermittent problems... Also a smoke test of all vacuum lines/intake might reveal a problem.

 

We recently had a 996 owner/Youtuber that went though this diligently and tested/replaced many many things to finally install an aftermarket check valve in the fuel pressure line to keep the fuel from bleeding back into the fuel tank, and that solved his problem..

You can search for the thread and do all the tests he did, and even replace the things he did in order to finally fix the problem...I can't remember his name , it was Finch or Rinch or something, maybe someone can give more clues to find the thread, and his channel is "this Tiny Garage" or something like that..

That thread covers every "hard to start hot" scenario known....

 

Thanks for posting your result.
This is the video porschetech3 is referring to A fairly easy test to indicate faulty check valve at the fuel pump is to bridge terminals 87 and 30 (+12V battery voltage, always live) of the fuel pump relay after 45min before you turn the key to start the engine. If it starts like the cold engine, then it is indeed the check valve.
Easiest way to bridge 87 and 30 is to remove the cap of the relay and stick a thick enough feeler gauge blade between coil and 'lever'

 

Thanks everyone, I’ll come back and let everyone know how it turns out.

 

Hi, ok start problem when hot, after about 45 minutes from shutdown fixed.
It was the pipe between the Fuel Pressure Regulator and the intake plenum which had snapped while I was changing the fuel injectors.

 

Thanks for the update.
Did you motice anything that could explain why the effect of not having the FPR referenced to the intake vacuum is so different between the warm engine sitting for 30 minutes and it sitting 45 minutes?
It almost sounds like the vacuum line held was able to hold some vacuum for about 30min but after another 15min of heatsoak it held nill and ambient pressure was what the fpr saw.

 

Actually I thought about that and need to get a pressure tester to check the bleed off in the fuel rail over time. However I assumed it had something to do with me dropping the engine and lifting the rail when changing the injectors because everything was fine before I started the job. I did disconnect the tube at the FPR prior to starting the job, but you know with the limited space and with all the pulling and pushing you can disturb a number of things.